Thread和Object类详解

方法概览

Thread

wait、notify、notifyAll方法详解

作用

阻塞阶段

使用了wait方法之后，线程就会进入阻塞阶段，只有发生以下四种情况中的其中一个，线程才会被唤醒

1. 另一个线程调用了这个线程的notify方法，刚好唤醒的是本线程
2. 另一个线程调用了这个对象的notifyAll方法
3. 过了wait规定的超时时间
4. 线程调用了interrupt

唤醒阶段

notify会唤醒单个处于阻塞状态的线程，唤醒的线程是随机的

notify和wait都需要写在synchronized代码块里，不然会抛出异常

notifyAll会唤醒所有等待的线程

遇到中断

执行wait方法之后，被中断，会抛出InterruptedException这个异常

代码展示

• 展示wait和notify的基本用法
• 该代码执行wait方法之后会释放锁，然后thread2执行notify方法
• notify方法执行完毕之后，并没有立即释放锁，而是接着执行之后的代码,也就是打印“Thread2调用notify”这句话
• thread2执行完毕之后，会进行释放锁，thread1才会继续执行
• 在此期间，thread1虽然被唤醒，但是一直在等待thread2同步代码块里面的代码执行完毕

public class Wait {

    public static void main(String[] args) throws InterruptedException {
        Thread1 thread1 = new Thread1();
        Thread2 thread2 = new Thread2();
        thread1.start();
        Thread.sleep(200);
        thread2.start();
    }

    public static Object object = new Object();

    static class Thread1 extends Thread {
        @Override
        public void run() {
            synchronized (object) {
                System.out.println("Thread1执行");
                try {
                    object.wait();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("Thread1获取锁");
            }
        }
    }

    static class Thread2 extends Thread {
        @Override
        public void run() {
            synchronized (object) {
                object.notify();
                System.out.println("Thread2调用notify");
            }
        }
    }
}

/*
Thread1执行
Thread2调用notify
Thread1获取锁
* */

• notify和notifyAll的展示
• 第一个输出：threadc调用notifyAll
• 第二个输出：threadc调用notify
• 调用notify的时候，程序并没有结束，threadb陷入等待

public class notifyOrAll implements Runnable{

    private static final Object a = new Object();

    public static void main(String[] args) throws InterruptedException {

        Runnable r = new notifyOrAll();
        Thread threada = new Thread(r);
        Thread threadb = new Thread(r);
        Thread threadc = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (a) {
//                    a.notifyAll();
                    a.notify();
                    System.out.println(Thread.currentThread().getName() + "notify");
                }
            }
        });
        threada.start();
        Thread.sleep(200);
        threadb.start();
        Thread.sleep(200);
        threadc.start();

    }

    @Override
    public void run() {
        synchronized (a) {
            System.out.println(Thread.currentThread().getName() + "得到锁");
            try {
                System.out.println(Thread.currentThread().getName() + "wait");
                a.wait();
                System.out.println(Thread.currentThread().getName() + "wait结束");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

/*
Thread-0得到锁
Thread-0wait
Thread-1得到锁
Thread-1wait
Thread-2notifyAll
Thread-1wait结束
Thread-0wait结束
* */

/*
Thread-0得到锁
Thread-0wait
Thread-1得到锁
Thread-1wait
Thread-2notify
Thread-0wait结束
* */

• 只释放当前monitor
• 证明wait只释放当前的那把锁

public class OwnMonitor {

    private static volatile Object a = new Object();
    private static volatile Object b = new Object();

    public static void main(String[] args) throws InterruptedException {

        Thread threadA = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (a) {
                    System.out.println("threadA得到a");
                    synchronized (b) {
                        System.out.println("threadA得到锁b");

                        try {
                            System.out.println("threadA释放a");
                            a.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        });

        Thread threadB = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (a) {
                    System.out.println("threadB得到a");
                    System.out.println("threadB要获取b");
                    synchronized (b) {
                        System.out.println("threadB得到b");
                    }
                }
            }
        });

        threadA.start();
        Thread.sleep(1000);
        threadB.start();
    }

}

/*
threadA得到a
threadA得到锁b
threadA释放a
threadB得到a
threadB要获取b
* */

特点

• 执行这些方法必须先获取锁
• notify只能换取一个，而且是随机的
• 都属于Object。任何对象都可以调用
• 都是native final修饰的

当线程从wait状态刚被唤醒时，通常不能直接得到锁，那就会从waiting状态转换到blocked状态，抢到锁之后状态转变为runnable

如果发生异常，则直接跳到Terminated状态

通过wait、notify方法实现生产者和消费者

• 将storge当作生产者和消费者进行工作的仓库
• 如果storge中没有数据，生产者就开始wait
• 如果storge中数据满了，消费者就开始wait
• 生产者和消费者每进行一次生产和消费，就执行notify

public class ProducerConsumer {

    public static void main(String[] args) {

        Storge storge = new Storge();
        Producer producer = new Producer(storge);
        Consumer consumer = new Consumer(storge);
        new Thread(producer).start();
        new Thread(consumer).start();
    }
}

class Producer implements Runnable {

    private Storge storge;

    public Producer(Storge storge) {
        this.storge = storge;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            storge.put();
        }
    }
}

class Consumer implements Runnable {

    private Storge storge;

    public Consumer(Storge storge) {
        this.storge = storge;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            storge.take();
        }
    }
}

class Storge {

    private int maxSize;
    private LinkedList<Date> storge;

    public Storge() {
        maxSize = 10;
        storge = new LinkedList<>();
    }

    public synchronized void put() {
        while (storge.size() == maxSize) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        storge.add(new Date());
        System.out.println("已经有了" + storge.size());
        notify();
    }

    public synchronized void take() {
        while (storge.size() == 0) {
            try {
                wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        System.out.println("拿到了" + storge.poll() + "还剩" + storge.size());
        notify();
    }
}

sleep方法详解

作用：让线程在预期的时间执行，其他时间不占用CPU资源

特点：和wait不一样，sleep不释放锁

sleep不会释放锁

• 证明sleep不会释放 synchronized锁

public class SleepSyn implements Runnable{

    public static void main(String[] args) {
        SleepSyn sleepSyn = new SleepSyn();
        new Thread(sleepSyn).start();
        new Thread(sleepSyn).start();
    }

    @Override
    public void run() {
        syn();
    }

    private synchronized void syn() {
        System.out.println(Thread.currentThread().getName() + "获取锁");
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + "释放锁");
    }
}

/*
* Thread-0获取锁
Thread-0释放锁
Thread-1获取锁
Thread-1释放锁
* */

• 证明sleep不释放Lock锁

  public class sleepLock implements Runnable{
  
      private static final Lock LOCK = new ReentrantLock();
  
      @Override
      public void run() {
          LOCK.lock();
          System.out.println(Thread.currentThread().getName() + "获取锁");
          try {
              Thread.sleep(2000);
          } catch (InterruptedException e) {
              e.printStackTrace();
          } finally {
              LOCK.unlock();
          }
          System.out.println(Thread.currentThread().getName() + "释放锁");
      }
  
      public static void main(String[] args) {
          sleepLock sleepLock = new sleepLock();
          new Thread(sleepLock).start();
          new Thread(sleepLock).start();
      }
  }
  
  /*
  * Thread-0获取锁
  Thread-0释放锁
  Thread-1获取锁
  Thread-1释放锁
  * */
  

sleep响应中断

• 抛出InterruptedException
• 会清除中断状态
• 中断之后，抛出异常继续执行

public class sleepInterrupted implements Runnable{

    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new sleepInterrupted());
        thread.start();
        Thread.sleep(2000);
        thread.interrupt();

    }

    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println(new Date());
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                System.out.println("中断");
                e.printStackTrace();
            }
        }
    }
}

/*
* Fri Jan 27 21:11:57 CST 2023
Fri Jan 27 21:11:58 CST 2023
中断
Fri Jan 27 21:11:59 CST 2023
java.lang.InterruptedException: sleep interrupted
   at java.lang.Thread.sleep(Native Method)
   at java.lang.Thread.sleep(Thread.java:340)
   at java.util.concurrent.TimeUnit.sleep(TimeUnit.java:386)
   at com.jx.JavaTest.ThreadObjectMethod.sleepInterrupted.run(sleepInterrupted.java:21)
   at java.lang.Thread.run(Thread.java:748)
Fri Jan 27 21:12:00 CST 2023
Fri Jan 27 21:12:01 CST 2023
Fri Jan 27 21:12:02 CST 2023
Fri Jan 27 21:12:03 CST 2023
Fri Jan 27 21:12:04 CST 2023
Fri Jan 27 21:12:05 CST 2023
Fri Jan 27 21:12:06 CST 2023

Process finished with exit code 0

* */

总结

sleep方法可以让线程进入waiting状态，不占用CPU资源，但是不释放锁，规定时间之后再运行

休眠期间如果被打断，会抛出异常并清除中断状态

join方法详解

新线程加入，主线程等子线程执行完毕

代码展示

• 前一个结果是使用join
• 后一个结果是没使用join
• 可知使用join之后，主线程会等join的线程执行完毕再继续执行

public class join {
    public static void main(String[] args) throws InterruptedException {
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "执行完毕");
            }
        });

        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "执行完毕");
            }
        });

        thread1.start();
        thread2.start();
        System.out.println("开始等待子线程运行");
//        thread1.join();
//        thread2.join();
        System.out.println("所有线程执行完毕");
    }
}

/*
* 开始等待子线程运行
Thread-0执行完毕
Thread-1执行完毕
所有线程执行完毕
* */

/*
* 开始等待子线程运行
所有线程执行完毕
Thread-1执行完毕
Thread-0执行完毕
* */

• 遇到中断
• 第一个的运行结果是主线程没中断的打印结果
• 第二个的运行结果是join期间进行中断的打印结果，可知在打印了“子线程运行完毕”之后，依然打印了“启动”两个字，可知会造成运行混乱
• 可以在捕获异常的代码块中，将join的线程也中断，可以解决上面的问题

public class joinInterrupt {
    public static void main(String[] args) {
        Thread main1 = Thread.currentThread();
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    main1.interrupt();
                    Thread.sleep(2000);
                    System.out.println("启动");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        thread1.start();
        System.out.println("join");
        try {
            thread1.join();
        } catch (InterruptedException e) {
            System.out.println(Thread.currentThread().getName() + "中断");
            // thread1.interrupt();
            e.printStackTrace();
        }
        System.out.println("子线程运行完毕");
    }
}


/*
* join
启动
子线程运行完毕
* */

/*
* join
main中断
子线程运行完毕
java.lang.InterruptedException
   at java.lang.Object.wait(Native Method)
   at java.lang.Thread.join(Thread.java:1252)
   at java.lang.Thread.join(Thread.java:1326)
   at com.jx.JavaTest.ThreadObjectMethod.joinInterrupt.main(joinInterrupt.java:23)
启动

Process finished with exit code 0

* */

/*
* join
main中断
子线程运行完毕
java.lang.InterruptedException: sleep interrupted
   at java.lang.Thread.sleep(Native Method)
   at com.jx.JavaTest.ThreadObjectMethod.joinInterrupt$1.run(joinInterrupt.java:13)
   at java.lang.Thread.run(Thread.java:748)
java.lang.InterruptedException
   at java.lang.Object.wait(Native Method)
   at java.lang.Thread.join(Thread.java:1252)
   at java.lang.Thread.join(Thread.java:1326)
   at com.jx.JavaTest.ThreadObjectMethod.joinInterrupt.main(joinInterrupt.java:23)

Process finished with exit code 0

* */

• join期间，线程处于WAITING状态

  public class joinStates {
  
      public static void main(String[] args) throws InterruptedException {
          Thread main1 = Thread.currentThread();
  
          Thread thread = new Thread(new Runnable() {
              @Override
              public void run() {
                  try {
                      Thread.sleep(3000);
                      System.out.println(main1.getState());
                      System.out.println("子线程运行结束");
                  } catch (InterruptedException e) {
                      e.printStackTrace();
                  }
              }
          });
  
          thread.start();
          System.out.println("join");
          thread.join();
          System.out.println("运行完毕");
  
      }
  }
  
  /*
  * join
  WAITING
  子线程运行结束
  运行完毕
  * */
  

yield方法

用来释放CPU时间片，但是不一定能达到预期的效果，因为有时CPU资源不紧张，无需yield

和sleep的区别是：sleep期间不会被再次调度但是yield会立刻处于竞争状态，还会随时再次被调度